Network elements that at least partially perform path routing functions of a switching computer are referred to also as network node elements. One widely familiar network node element is what is termed the router.
A router conveys arriving data packets over further network node elements or routers to a provided network element in a destination network requiring to be determined. A router determines an optimal path to the destination network element when data packets arrive, with said path possibly traversing further network node elements or routers. A suitable interface via which the data packets are to be forwarded is in connection therewith selected in the router. For that purpose the router utilizes a locally provided routing table. The entries in said table can be static or dynamic. Static entries are made by a network administrator, whereas dynamic entries are created and modified during ongoing operation. The dynamic entries are therein created and modified using what are termed routing protocols.
Routing protocols serve to exchange routing information; they allow the routers to structure their routing tables dynamically and to adjust them in response to altered conditions. A major instance of altered conditions is therein a complete or partial outage of the router. An outage of said type of a neighboring router is referred to also as a “neighbor down”. A partial outage can affect, for example, a single interface of the router whose other interfaces remain intact, whereas a complete outage will disable all the router's functions.
The term “outage” below encompasses both partial complete outages. The term “router” is for ease of understanding furthermore applied below to network node elements in general without thereby restricting what is under consideration to a router's specific embodiment.